It has been known for some time that it would be of advantage to mount a power steering unit on the axle of a vehicle, especially on large trucks with a one-piece front axle. So mounted, the associated steering linkages and rods do not move up and down relative to the power steering unit as the vehicle is driven, as occurs in trucks with frame steering gears. That is, with an axle mount gear unit, as the wheels of the vehicle and the supporting axle move to negotiate bumps in the road, the power steering unit moves also. There is thus eliminated the constant change in the angle of the drag link connected to the crank of the power steering unit that would otherwise occur with each up and down movement of the wheels, and of course that would occur also due to different loading on the springs of the vehicle.
As the need, and the desirability, of using dual power steering units on a truck or similar vehicle has arrived, it has become even more desirable to develop the axle mount. This is so, since while the need to avoid changes in the geometry of the steering linkage is important in a single unit installation, it is of paramount importance in a dual system. Each change in geometry in a dual system where the units are frame mounted introduces a force mismatch causing "fighting". This lowers steering efficiency and has the potential of causing steering "chatter".
Another factor pointing toward development of a successful axle mount, is that recently there has been developed a reliable extensible and flexible connection for connecting the steering shaft of the vehicle to the input shaft of the power steering unit. This extensible and flexible connection absorbs the road shock that previously had to be accommodated in the movement of the steering drag link, and does so without adversely affecting the operation of the steering system. Finally, the technology of building steering gears has developed to the point where the steering unit is not adversley affected by the road shock that it must bear as it rides on the axle.
Mounting of the power steering unit on the axle appears simple at first blush, but is complicated by the very high reaction forces that must be accommodated as the power steering gear operates. Another important consideration is the need for mounting of the unit on the truck so as to be orientated approximately the same as before. This minimizes other changes in engineering of the vehicle for obvious economic benefits. Also, for several years, every available cubic inch of space in and around the power plant compartment has been utilized for auxiliary equipment. Moving the steering gear down adjacent the axle frees premium space above and makes service of all components easier.
A natural location for an axle mounted power steering gear unit is adjacent the spring on the side of the vehicle where the operator is located. If a dual system is employed, the slave or second unit is simply mounted in the same relative location on the opposite side of the vehicle. One reason this is an ideal location is that the axle is reinforced at the place where the spring connects. In other words, a mounting pad is already provided to accommodate the spring. Mounting the power steering unit on a bracket sandwiched between the spring pad and the spring has been found to be highly advantageous.
With the selection of the ideal location, the main problem left to be solved, as indicated above, is how the bracket is to be securely locked in position on the pad to resist the powerful reaction forces of the steering unit. Early attempts have included making a special casting for the power steering unit with the locking lip or tongue on the casting. A special upper spring mounting block is provided with a cooperating grooved extension mating with the tongue on the casting. Thus, the previous design concept was to have a tongue and groove connection above the spring. The draw back of this design is that substantial reaction forces must thus be resisted by the conventional U-bolt clamps that hold the mounting block and extend around the spring leaves and down through the mounting pad on the axle.
Basically, the special tongue and groove connection transmits the reaction forces through to the U-bolts that must then provide the full resistance of the steering unit.
Especially in large vehicles, such as heavy trucks, the previous concept has thus proven to leave much to be desired. The length of each leg of the U-bolt from the special upper spring block to the pad is substantial since the spring leaves pass between them. These U-bolts, because of the lever arm effect, are thus subject to twisting as the gear operates. Especially, over an extended period of operation, the U-bolts bend and elongate. Eventually, due to fatigue, looseness may develop and failure of the mounting inevitably follows.